issue: #34715
if collection's segment list doesn't changes anymore, then the next
target will be empty at most time, and balance segment will check
whether segment exist in both current and next target, so the balance
cloud be blocked due to next target is empty.
This PR permit segment to be moved if next target is empty, to avoid
balance stuck.
Signed-off-by: Wei Liu <wei.liu@zilliz.com>